In a conventional package-on-package (POP) process, a top package, in which a first device die is bonded, is further bonded to a bottom package through solder balls. The bottom package may also include a second device die bonded therein. The second device die may be on the same side of the bottom package as the solder balls.
Before the bonding of the top package to the bottom package, a molding compound is applied on the bottom package, with the molding compound covering the second device die and the solder balls. Since the solder balls are buried in the molding compound, a laser ablation or drilling is performed to form holes in the molding compound, so that the solder balls are exposed. The top package and the bottom package may then be bonded through the solder balls in the bottom package. In addition, the molding compound may be filled into the space between the second device die and a package substrate in the bottom package. Since the space between the second device die and the package substrate in the bottom package is small, a void may be adversely generated in the space, which results in stress to the second device die.